Exotic spinlocks: X_PACKET_LIST and XID_ARRAY

Exotic spinlocks: X_PACKET_LIST and XID_ARRAY

A Twitter conversation sparked this blog post; the question there was about the relevance of the X_PACKET_LIST spinlock. As with most spinlocks, there is no public documentation on this one, and that is primarily because the spinlock is an implementation detail and can change from release to release.

‘Do it yourself’

That said, we can uncover some basic details about this spinlock. As preliminary reading, I suggest that you read Craig Freedman’s post on the way parallelism is implemented in SQL Server. Next, it may help if you can get handy with a debugger and SQL Server public symbols, something on the lines of this SQLCAT blog post. Another useful background read is Paul White’s post where he alludes to some of the architecture elements behind parallelism.

Now, I am going to simplify things within the limits of what we can disclose publicly. FYI, the information I share below can be obtained by anyone with a little bit of WinDbg magic and co-relating to the sys.dm_xe_map_values DMV. For example, if you look at the call stack in the SQLCAT team blog post and later poke around in the debugger with some breakpoints and the x (Examine Symbols) command, you can deduce the following:

An exchange (parallelism) operation has CXPipe instances. Conceptually think of these as the ‘wiring’ between producers and consumers within a parallel exchange (see sqlmin!CXPipe::NewRow and sqlmin!CXPipe::GetRow in the debugger)

These pipes have a CXPacketMgr class which in turn has a CXPacketList. From the name it suggests that this is a data structure into which producers insert (sqlmin!CXPacketList::Append in the debugger) and consumers pull packets from (sqlmin!CXPacketList::RemoveHead in the debugger.)

Any shared data structure must be protected for this kind of concurrent access, and the way we chose to do it in SQL is to use a spinlock – the X_PACKET_LIST spinlock.

Connecting the dots

Now, in Christian’s case (from the Twitter thread) he was also observing relatively high spins and backoffs for XID_ARRAY spinlock. That one is also related to the parallelism implementation. To explain this, there are two other classes of interest: a CXPort class and a CXId (Exchange ID for each worker). For simplicity, think of the CXPort having entries for the CXIds (representing worker threads) in the form of an array. Access to that array is protected using the XID_ARRAY spinlock.

Conclusion

So in conclusion, both these spinlocks have everything to do with parallelism. Experimenting with different values for MAXDOP may be beneficial, and also check if there are excessive amounts of data (LOB data maybe) flowing in the exchange. Beyond that, if you are in doubt or clueless, please do not hesitate to contact Microsoft Customer Support Services (CSS) and log an incident to debug the root cause.